A light receiving circuit that is mounted in, for example, a photocoupler is widely used in electronic devices for industry, for communication, for households, etc.
In such an electronic device, a signal is transmitted between different power supply systems such as a DC voltage system, an AC power supply system, a telephone line system, etc.
By using the photocoupler, an electrical signal can be transmitted in a state in which the different power supply systems are insulated.
The photocoupler includes the light receiving circuit that changes the optical signal into the electrical signal. In the case where the light receiving circuit includes a MOS transistor, the cut-off frequency of the transimpedance amplifier can be increased and a high speed response is easy to realize by increasing the transconductance.
However, the capacitance of the MOS transistor becomes large; the switching time of the current comparator that has a cascade connection to the transimpedance amplifier becomes long; and such a configuration is not quite sufficient to increase the operation speed of the entire light receiving circuit.
Also, in the case where the reverse voltage of the light receiving element is low, the operation speed of the entire light receiving circuit decreases due to the higher junction capacitance.